This invention relates to a developing unit of dry type, and in particular, to such developing units which may be used in an electrophotographic machine or electrostatic recorder.
In an electrophotographic machine or electrostatic recorder, an electrostatic latent image is formed on the surface of a recording member such as a photosensitive or dielectric member according to a predetermined technique. Such recording members can be referred to as latent image carriers. The latent image formed on such carriers is developed with a colored fine powder, referred to as toner, supplied from a developing unit. The toner is usually charged to the opposite polarity from the latent image, whereby it is electrostatically attracted to the latent image.
To charge the toner to the opposite polarity from the latent image, the developer commonly comprises a mixture of toner and carrier. The toner is triboelectrically charged by mixing with and stirring with the carrier. Such developer is referred to as two-component developer. Since only the toner is consumed during the developing process, the proportions of the toner and carrier vary at various points of the developing process and during each developing process. If the relative proportions are allowed to vary in such manner, the toner concentration varies from point to point of the developing process, resulting in a non-uniform density of the image being developed. For this reason, it will be seen that a sufficient mixing and stirring of the developer must be made.
To achieve a uniform toner concentration, the stirring action applied by the developing unit must be sufficiently made, in particular, in the direction across the image carrier. An ordinary latent image formed on the image carrier has different distributions of charge in the crosswise and the lengthwise direction, resulting in different amounts of toner consumption in the respective directions. Any non-uniformity in the toner consumption in the lengthwise direction of the image carrier can be levelled out during the circulation of the developer through the developing unit. By contrast, a non-uniformity in the toner consumption in the crosswise direction of the image carrier remains unchanged unless it is positively eliminated or reduced.
A conventional developing unit of dry type which is found in the prior art is provided with additional means for providing a developer stirring action in the crosswise or transverse direction. Such means may comprise a screw having a helical blade, an impeller wheel having a number of discs which are disposed at an angle with respect to the rotary shaft thereof, or a deflector plate for changing the direction of flow of the developer. To achieve an effective stirring action, the developer which has once been directed tranversely in one direction by such members must be returned back in the opposite direction. Accordingly, when the developer is stirred with an agitator of impeller wheel, a pair of impeller wheels are provided and arranged for reverse rotation. Such arrangement result in a complex mechanism and require an increased magnitude of torque. When a deflector plate is employed to stir the developer, there must be provided a plurality of deflector plates in the course of free fall or flow under gravity, resulting in a bulky arrangement.